More helpful error message for ExtensionList.lookupSingleton

[jglick]

A periodic issue, most recently noticed going over something with @julieheard: if you run JenkinsRule tests using a special test runner from an IDE which does not autodiscover annotation processors, things will be badly broken, and a typical symptom is this otherwise fairly opaque error. If in doubt, use Maven to run tests (and invoke the clean phase to be sure).

Testing done

None, my IDE runs Maven (well, mvnd) to begin with! Edit: #8646 (comment)

Proposed changelog entries

• N/A

Before the changes are marked as ready-for-merge:

Maintainer checklist

Preview Give feedback

1. There are at least two (2) approvals for the pull request and no outstanding requests for change.
2. Conversations in the pull request are over, or it is explicit that a reviewer is not blocking the change.
3. Changelog entries in the pull request title and/or Proposed changelog entries are accurate, human-readable, and in the imperative mood.
4. Proper changelog labels are set so that the changelog can be generated automatically.
5. If the change needs additional upgrade steps from users, the upgrade-guide-needed label is set and there is a Proposed upgrade guidelines section in the pull request title (see example).
6. If it would make sense to backport the change to LTS, a Jira issue must exist, be a Bug or Improvement, and be labeled as lts-candidate to be considered (see query).

[jglick]

Found a way to test, sort of. After

mvnd -am -pl war -Pquick-build install
cd ../branch-api-plugin

and

diff --git src/main/java/jenkins/branch/WorkspaceLocatorImpl.java src/main/java/jenkins/branch/WorkspaceLocatorImpl.java
index 4515492..14d452b 100644
--- src/main/java/jenkins/branch/WorkspaceLocatorImpl.java
+++ src/main/java/jenkins/branch/WorkspaceLocatorImpl.java
@@ -82,7 +82,6 @@ import org.kohsuke.accmod.restrictions.NoExternalUse;
  * @see "JENKINS-2111"
  */
 @Restricted(NoExternalUse.class)
-@Extension(ordinal = -100)
 public class WorkspaceLocatorImpl extends WorkspaceLocator {
 
     private static final Logger LOGGER = Logger.getLogger(WorkspaceLocatorImpl.class.getName());

Before:

$ mvn test -Dtest=WorkspaceLocatorImplTest\#locate
…
   4.380 [id=39]	INFO	jenkins.InitReactorRunner$1#onAttained: Completed initialization
   4.413 [id=20]	WARNING	jenkins.model.Jenkins#<init>: Exception in onOnline() for the computer listener class jenkins.branch.WorkspaceLocatorImpl$Collector on the built-in node
java.lang.IllegalStateException: Expected 1 instance of jenkins.branch.WorkspaceLocatorImpl but got 0
	at hudson.ExtensionList.lookupSingleton(ExtensionList.java:454)
	at jenkins.branch.WorkspaceLocatorImpl.lockFor(WorkspaceLocatorImpl.java:310)
	at jenkins.branch.WorkspaceLocatorImpl$Collector.onOnline(WorkspaceLocatorImpl.java:631)
	at jenkins.model.Jenkins.<init>(Jenkins.java:1026)
	at hudson.model.Hudson.<init>(Hudson.java:86)
	at org.jvnet.hudson.test.JenkinsRule.newHudson(JenkinsRule.java:697)
	at org.jvnet.hudson.test.JenkinsRule.before(JenkinsRule.java:406)
	at org.jvnet.hudson.test.JenkinsRule$1.evaluate(JenkinsRule.java:601)
	at org.junit.internal.runners.statements.FailOnTimeout$CallableStatement.call(FailOnTimeout.java:299)
	at org.junit.internal.runners.statements.FailOnTimeout$CallableStatement.call(FailOnTimeout.java:293)
	at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
	at java.base/java.lang.Thread.run(Thread.java:829)
…
ERROR] Failures: 
[ERROR]   WorkspaceLocatorImplTest.locate:157 expected:<…/branch-api-plugin/target/tmp/j h16047186359448965079/workspace/stuff_dev_flow> but was:<…/branch-api-plugin/target/tmp/j h16047186359448965079/workspace/stuff/dev%2Fflow>
[INFO] 
[ERROR] Tests run: 1, Failures: 1, Errors: 0, Skipped: 0
…

After:

$ mvn test -Dtest=WorkspaceLocatorImplTest\#locate -Djenkins.version=2.430-SNAPSHOT
…
java.lang.IllegalStateException: Found no instances of jenkins.branch.WorkspaceLocatorImpl registered (possible annotation processor issue); try using `mvn clean test -Dtest=…` rather than an IDE test runner
…

[NotMyFault]

/label ready-for-merge

---

This PR is now ready for merge. We will merge it after ~24 hours if there is no negative feedback.
Please see the merge process documentation for more information about the merge process.
Thanks!